1. Technical Field
This invention relates to telephone conferencing arrangements and, more particularly, to voice-switched telephone conferencing arrangements employing bandstop and bandpass filters.
2. Description of the Prior Art
Telephone conferencing arrangements with multiple parties participating at a telephone station generally use voice-switched circuits to avoid undesirable acoustic coupling between a receiver and a microphone located together at the station.
In a loop comprising two "hands-free" stations, for example, there can be substantial direct acoustic coupling between the local receiver and the transmitter at each station. If at any time the net loop gain is greater than unity, the loop becomes unstable and may oscillate. The undesirable speech signal reoccurrence can be viewed as a speaker's voice returning to his own transmitter via the two direct acoustic paths, one at the remote station and the other at the talker's station.
In such loops, even when overall gain is low, there still is the problem of remote end echo, which stems from a speaker's voice returning to his ear, at a reduced but discernible level, after traveling around such a loop. For remote end echo to occur requires only one hands-free station in the loop; and both the direct and indirect acoustic paths at such station contribute to the echo.
In multipoint telephone conferencing arrangements, additional voice switching often takes place in a conference bridge which interconnects the lines from the stations and provides amplification to compensate for losses in a switched network. Hybrid echoes from the multiple paths are also present in the bridge and are minimized by attenuation circuits which place loss in a receive path whenever there is a signal in an associated send path. Thus if a person at one location talks, attenuation is placed in his receive path at the bridge. And a voice gate in the send path quiets the line in the idle state when no one is talking.
While the above arrangements have been found satisfactory in providing a stable, quiet and good level telephone conference connection, they do not allow more than one party to speak at any given time. In an effort to overcome this disadvantage and avoid the echo and feedback problems, the use of complementary comb filters was proposed by D. A. Berkley and J. S. Courtney-Pratt in U.S. Pat. No. 3,622,714, issued on Nov. 23, 1971. In this patent, two banks of comb filters are utilized on a voice channel with the filter passbands of the first bank being stopbands in the second bank and vice versa. The two filter banks are incorporated into two hands-free telephone stations, with the signal received including frequencies only within the bandpass set of the sending station. As the two passband sets are mutually exclusive, no closed feedback path exists in the loop to cause instability.
While this arrangement proved satisfactory for a conference involving two hands-free telephone stations, it is not readily adaptable to multiparty telephone conferencing.